If you're not super heavy into PC gaming, don't do it yet. 1440p with higher frame rates is your friend. To utilize a 4K monitor with high fps, you need hardware that either doesn't exist or simply costs so much that it blows your budget into the stratosphere. A decent 4K monitor alone will cost more than any console, or an entire decent PC. For 1440p though, I'd recommend either 27" or even 32", I'm currently using a 32" display and it has served me well for many years. You can get a really good 1440p monitor for a fraction of the price of a 4K panel.
He's right. 1440p is the current sweetspot, since you get a step up from 1080p in res bump and still able to target higher frame rates. For me I'm able to go for higher frame rates with my 1080ti and still go to max settings for most games.
4k still seems like a waste atm. I'll end up jumping to 4k when monitors are way cheaper than what they are currently at, as well as games sporting more than just 4k textures on character/weapon models.
I'd just go 1440p. You can save money, get higher frame rates and if you really wanna do some 4k, you can downsample if need be.